since this makes it trigger immediately without having to wait for her mouth and there are no other competing phrases.

I used the auto-mode for a good 10 minutes, making sure to restart it any time he was just holding or otherwise not pounding away at full speed. I got maybe 2-3 vigorous lines! Is that normal?

What are the exact criteria to trigger vigorous lines? My observations:
*DEPTH seems to be the most important factor. He can push it past her gag reflex several times rapidly, but unless he really pounds her skull into his crotch while doing it, it won't be "vigorous". The last inch really counts, which was a surprise to me.
*Number of times is a factor. I tried to count the really long consecutive strokes, and believe I saw it trigger at around 8-10 times. I'm pretty sure 4-6 times is not enough, even if they are deep and fast.
*Speed matters. I tried to replicate precise deep strokes in perfect succession, but couldn't do it fast enough to trigger vigorous manually. During other testing, I've managed to trigger vigorous lines once or twice manually during the last week or so of daily play.

Is it supposed to be this difficult? If anyone is triggering vigorous lines with ease, please let me know your secret :)

Using the Loader, you can see the numerical value of your roughness (top of the window, along with his arousal, her air, and how much cum in her mouth). Vigorous doesn't trigger until the value is pretty close to 100, but I have no problem triggering it.